Public Advocate Bill de Blasio is leading New York City's Democratic primary race for mayor, with 43 percent support among Democrats likely to vote -- more than the 40 percent that would be needed to avoid a runoff election -- according to a Quinnipiac University poll released Tuesday.

He is trailed by former Comptroller Bill Thompson at 20 percent, City Council Speaker Christine Quinn at 18 percent, former Rep. Anthony Weiner at 7 percent and Comptroller John Liu at 4 percent.

The poll showed de Blasio topping Quinn -- the race's only female candidate -- among female voters, 44 percent to 18 percent, and leading Thompson -- the only African-American contender -- with black voters, 47 percent to 25 percent.

The poll surveyed 750 likely Democratic voters between Aug. 28 and Sept. 1 and has a margin of error of plus or minus 3.6 percentage points.

"The momentum and the trajectory are really great" for de Blasio, said Quinnipiac poll director Maurice Carroll. If no candidate gets at least 40 percent of the primary vote on Sept. 10, the first- and second-place finishers meet in a runoff on Oct. 1.

An Aug. 29 amNewYork-News 12 poll of likely Democratic primary voters had de Blasio leading with 29 percent, Thompson close behind with 24 percent and Quinn trailing with 17 percent. That survey was conducted Aug. 22-27.

De Blasio has sought to separate himself from the pack by billing himself as the most progressive candidate, supporting two City Council measures to reform stop and frisk and pledging to raise taxes on those making $500,000 or more. He enjoys support from 54 percent of voters who consider themselves "very liberal," the poll showed.

In potential runoff scenarios, de Blasio would top Quinn, 66 percent to 25 percent; de Blasio would defeat Thompson, 56 percent to 36 percent; and Thompson would beat Quinn, 59 percent to 33 percent.

Eight percent of voters said they are undecided, and 24 percent said they could change their minds in the final week of the primary campaign -- an indication of the race's volatility.

In the GOP race, former MTA chairman Joe Lhota had 48 percent support among likely Republican primary voters, double the 24 percent for supermarket and oil magnate John Catsimatidis. Philanthropist George McDonald has 10 percent. The poll surveyed 101 likely Republican voters, with a margin of error of plus or minus 9.8 percentage points.

The amNewYork-News 12 poll last week found Lhota leading Catsimatidis by 50 percent to 28 percent, with 5 percent for McDonald.